1. Field of the Invention
The present invention relates to a method for manufacturing a surface acoustic wave apparatus, and more particularly, the present invention relates to improvements in properties of an electrode film included in a surface acoustic wave apparatus for use as, for example, a resonator and a filter.
2. Description of the Related Art
Regarding electronic apparatuses, increases in frequencies and improvements in functions and performances have been accelerated. In addition, devices included in the electronic apparatuses have also been required to have increases in frequencies and improvements in functions and performances. In addition, techniques for manufacturing such a device having high quality functions and performances at lower cost have been also intensely required.
Regarding the surface acoustic wave apparatus using a surface acoustic wave, apparatuses using Rayleigh waves and apparatuses using Love waves are known. These surface acoustic wave apparatuses are manufactured by a method including the steps of forming electrode films by vapor deposition on piezoelectric substrates made of quartz or LiNbO3, and subsequently, by patterning the electrode films into predetermined electrode shapes via photolithographic etching so as to form interdigital electrode transducers on the substrates. Thereafter, the device properties and characteristics of the surface acoustic wave apparatuses are evaluated.
In these two surface acoustic wave apparatuses, the surface acoustic wave apparatuses using Rayleigh waves have been widely used. As the electrode films, aluminum films made of aluminum or aluminum with copper additives, have been primarily used. With surface acoustic wave apparatuses using Rayleigh waves, however, it is difficult to miniaturize the devices due to small electromechanical coupling coefficients, small reflection coefficients, and other factors.
The aforementioned defects of the surface acoustic wave apparatuses using Rayleigh waves have been overcome by surface acoustic wave apparatuses using Love waves. With surface acoustic wave apparatuses using Love waves, as the materials for the electrode films, gold, tantalum, and tungsten, each having a high density, have been used. In surface acoustic wave apparatuses, the device properties must be stabilized, so that the stability and the reliability of the electrode films are improved. Therefore, among the aforementioned materials used for forming the electrodes, tantalum is used in many surface acoustic wave apparatuses using Love waves.
In conventional surface acoustic wave apparatuses using tantalum for the electrode films, the device properties, such as resonant frequencies, vary with time due to changes in specific resistances of the tantalum electrode films. The changes in the device properties are also caused by the thermal loads occurring during the passage through reflow furnaces after the surface acoustic wave apparatuses are manufactured.
Regarding the surface acoustic wave apparatus used as a resonator and a filter, the change in the resonant frequency is fatal, and when the change exceeds standards, the product is determined to be defective. As a consequence, in the manufacture of the surface acoustic wave apparatus, there are problems with preventing the device properties, such as the resonant frequency, from changing during the various manufacturing steps, and there are problems with improving the reliability of the surface acoustic wave apparatus.
As a method for stabilizing the specific resistance of the tantalum electrode film, the method in which the surface of the electrode film is covered with an anodized film is known.
Regarding tantalum, two kinds of tantalum, i.e., xcex1 tantalum and xcex2 tantalum, are present depending on the difference in the crystal structures. In these two types of tantalum, xcex2 tantalum is a superior resistive material exhibiting small changes in resistivity over time. On the other hand, it is reported in xe2x80x9cSputter Thin Film: Foundation and Applicationxe2x80x9d by Haruhiro Kobayashi, published by THE NIKKAN KOGYO SHIMBUN, LTD., that xcex1 tantalum has a small specific resistance compared to that of xcex2 tantalum, although the temperature stability of the resistivity is inferior and the adhesion force relative to the substrate is also inferior.
Therefore, xcex2 tantalum exhibiting superior stability was used as the resistive material for an electrode, the electrode film made of xcex2 tantalum was covered with an anodized film, and subsequently, the electrode film was subjected to a heat treatment in order to diffuse oxygen in air, and in order to prevent a distortion status of the electrode film from changing due to the heating of a resistor. As a method for oxidizing the surface of the electrode film made of xcex2 tantalum, a method in which a heat treatment is performed in air for oxidization, and a method in which a laser light is used for the oxidization are also known.
Even when the aforementioned method was applied, in which the electrode film was covered with the oxidized film, the stability of the device properties was insufficient in practice. Furthermore, when the tantalum electrode film was covered with the anodized film, the specific resistance property was stabilized, although the value of the specific resistance was increased. As a consequence, the device properties were degraded, and the bonding performance was degraded. In addition, regarding the method in which the electrode film was covered with the anodized film, there were problems in that the process became complicated, and the cost of manufacturing the surface acoustic wave apparatus was expensive. Regarding the method in which the oxidation was performed in air, the efficiency of the manufacture was decreased due to a large time consumption, and regarding the method in which the laser light was used, the cost of the apparatus was expensive.
Accordingly, regarding surface acoustic wave apparatuses using tantalum for the electrode film, a method for manufacturing a stable electrode film at a low cost without degradation of the bonding performance and the specific resistance of the electrode film has been required but never developed.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for manufacturing a surface acoustic wave apparatus, in which a specific resistance of an electrode film is decreased, so that device properties and a bonding performance are excellent, and a stable electrode film can be manufactured with an inexpensive and simple process.
According to a preferred embodiment of the present invention, a method for manufacturing a surface acoustic wave apparatus preferably includes the steps of forming an electrode film including tantalum on a piezoelectric substrate in a vacuum, working the electrode film into a desired shape, and heat-treating the resulting electrode film at a temperature of from about 200xc2x0 C. to about 700xc2x0 C.
Regarding the aforementioned surface acoustic wave apparatus, by forming the electrode film that is preferably primarily composed of tantalum on the piezoelectric substrate, and subsequently, by heat-treating the resulting electrode film at a temperature of from about 200xc2x0 C. to about 700xc2x0 C., hydrogen occluded in the electrode film is removed. By removing the hydrogen from the electrode, the specific resistance of the electrode film is greatly decreased compared to that before the heat treatment, the stability and the reliability of the electrode film is greatly improved, and a highly reliable and low cost surface acoustic wave apparatus is produced.
When the heat treatment temperature exceeds about 700xc2x0 C., the effect of decreasing the specific resistance is hardly improved with the increase in the heat treatment temperature, and the piezoelectric substrate may be warped due to an increase in the stress on the electrode film. In order to produce the effect of decreasing the specific resistance due to the heat treatment, the heat treatment must be performed at a temperature of about 200xc2x0 C. or more, and the heat treatment is preferably performed at a temperature of about 300xc2x0 C. or more. Therefore, the heat treatment temperature of the tantalum electrode film is preferably within a range from about 200xc2x0 C. to about 700xc2x0 C., and more preferably, is within a range from about 300xc2x0 C. to about 700xc2x0 C.
Regarding the aforementioned method for manufacturing the surface acoustic wave apparatus according to a preferred embodiment of the present invention, the step of heat-treating is preferably performed in a vacuum or in an atmosphere not containing oxygen.
When the heat treatment is performed in a vacuum or in an atmosphere not containing oxygen, for example, in an atmosphere of an inert gas, the electrode film can be prevented from being oxidized, and the resistance of the electrode film can be prevented from being increased.
Regarding the aforementioned method for manufacturing the surface acoustic wave apparatus according to a preferred embodiment of the present invention, at least a part of the tantalum, as the primary component of the electrode film, is preferably composed of xcex1 tantalum.
Regarding tantalum, there are xcex1 tantalum and xcex2 tantalum, and both types of tantalum can be used for the electrode film. In particular, by using xcex1 tantalum, the specific resistance of the electrode film can be further decreased.
Regarding the aforementioned method for manufacturing the surface acoustic wave apparatus according to preferred embodiments of the present invention, the aforementioned step of heat-treating may be performed prior to the aforementioned step of working the electrode film.
Regarding the aforementioned method for manufacturing the surface acoustic wave apparatus according to preferred embodiments of the present invention, the aforementioned step of heat-treating may be performed between the step of working the electrode film and a step of evaluating properties of the surface acoustic wave apparatus.
The timing of the heat treatment of the electrode film may be prior to the step of working the electrode film into a desired shape, and may be after the step of working the electrode film into a desired shape. When the heat treatment is performed prior to the step of working, the specific resistance can be decreased compared to that in the case in which the heat treatment is performed after the step of working. When the heat treatment is performed after the step of working, the reliability of the electrode film is improved compared to that in the case in which the heat treatment is performed prior to the step of working. In addition, when the heat treatments are performed prior to the step of working and after the step of working, the specific resistance may be further decreased and the reliability is further improved.
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.